This invention relates to an apparatus for developing an electrostatic latent image, which has been generally employed in the field of electrophotography, and, more particularly, to a magnetic brush developing apparatus for use therein.
Conventionally, as a developer to be used for the above-described field is concerned, two-component developers such as a mixture constituted by magnetizable carrier particles, for example, iron particles, each having a relatively large particle-diameter (an approximate particle-diameter of 75 .mu.m) and non-magnetizable particles, or one-component developers employing magnetizable toner particles and the like are well known in the art. These developers inherently have a number of advantages, but nevertheless have been used in practical applications up to the present. Recently, in order to further improve the above-described developers, thereby to provide them with much improved characteristics, there has been considerable research which has resulted in somewhat different novel developers. More specifically, there have been proposed two-component developers respectively constituted by magnetizable particles and non-magnetizable particles having approximately the same particle diameters as each other, which developers are disclosed, for instance, in U.S. Patent application Ser. No. 863,616, filed Dec. 23, 1977, Susumu TANAKA et al., U.S. Pat. No. 4,202,491, in which a developer constituted by magnetizable toner and electrically insulating non-magnetizable toner at a predetermined mixing ratio by weight is employed as the developer, and also in U.S. Patent Application Ser. No. 949,426, filed Oct. 5, 1978, Kenji TABUCHI et al. in which there is employed a developer constituted by insulating toner particles and carrier particles having the properties of (1) being magnetic, (2) having a high electroresistivity (more than 10.sup.12 .OMEGA..multidot.cm), and (3) being 5 to 40 .mu.m in size.
Actually, although the respective, improved two-component developers as described above are capable of being used in a conventional magnetic brush developing apparatus without any important modification in the apparatus, some properties of the recently improved two-component developers described above are somewhat different from those of the conventional developers. Therefore, if the recently improved developers are employed for the developing process, the magnetic brush developing apparatus itself should be arranged to be applicable to such improved developers.
In order to be able to take advantage of the properties of such improved developers, there has been proposed by the present inventor a magnetic brush developing apparatus, as shown in FIG. 1, for which U.S. Patent application Ser. No. 74,812 was filed on Sept. 11, 1979 in the name of Kenji TABUCHI et al., which has not yet been laid open to public inspection in the U.S.A. and is different from the present invention which relates to an improvement thereof.
Referring to FIG. 1 showing the main portion of the magnetic brush developing apparatus disclosed in the above U.S. Patent application Ser. No. 74,812, the general construction and function thereof are briefly described hereinbelow.
The magnetic brush developing apparatus is enclosed by an upper casing 4, a lower casing 6 and a pair of side walls W which constitute a developing casing, and generally comprises a developing sleeve 2, and a multipolar magnet member 3 rotatably enclosed in the developing sleeve 2. The developing sleeve 2 has a cylindrical configuration and is made of non-magnetizable electrically conductive material such as aluminum and is disposed for rotation counterclockwise in the direction of the arrow "b" at 30 r.p.m. in a position close to the surface 1a of a photoreceptor drum 1 which is also capable of rotating counterclockwise in the direction of the arrow "a". The multipolar magnet member 3 has a roll-like configuration and has magnetic poles N and S sequentially arranged around the outer periphery with alternately different polar orientation as shown and is adapted to rotate at a high speed of 1,300 r.p.m. in the same direction as that of the rotation of the developing sleeve 2, i.e. in the direction indicated by the arrow "c" in FIG. 1. Accordingly, the developer is subjected to one certain specific moving speed in the direction of the arrow "b" due to the rotation of the developing sleeve 2 and also to another certain specific moving speed in the direction opposite that of the arrow "c" with respect to the developing sleeve 2 by the rotation of the multipolar magnet member 3 and is consequently moved along the peripheral surface of the developing sleeve 2 in the direction indicated by the arrow "d" at a speed which is the difference between the above-described two rotational speeds. In a position above the developing sleeve 2, there is provided a casing member 4a, which is suitably connected to the upper casing 4 so as to constitute a portion of the upper casing 4, and an inner surface 4b of the casing member 4a is shaped in an arcuate form so that the magnetic brush fromed on the peripheral surface of the developing sleeve 2 will effectively be in close contact with said inner surface 4b. Meanwhile, at a leading edge of the casing member 4a and on a line extending from the arcuate inner surface 4b, an electrically insulative sealing member 5 of resilient material is provided so as to contact the surface 1a of the photoreceptor drum 1. In a position below the developing sleeve 2, there are provided a lower enclosing member 7, a developer-trapping member 8, a supplementary cleaning member 9 the free end portion of which is directed in a direction opposite the direction of rotation of the developing sleeve 2, a scraping member 10 the free end portion of which is directed in a direction the same as the direction of rotation of the developing sleeve 2, a cleaning member 11 the free end portion of which is directed in a direction opposite the direction of rotation of the developing sleeve 2, and a bucket roller 12 having a plurality of trough-like members 12' provided around its peripheral surface for supplying the developer to the developing sleeve 2, and which is driven for rotation in the direction indicated by the arrow "e". The lower enclosing member 7 is impressed with a bias voltage having the same polarity as that of the electrostatic latent image, while the developer trapping member 8 is suitably grounded.
By the arrangement as described in the foregoing, the developer is first transported clockwise by the plurality of trough-like members 12' of the bucket roller 12, until the developer reaches a position (A) whereat the developer is affected by the magnetic force of the multipolar magnet member 3. At the position (A), the trough-like members 12' are turned over thereby to pour the developer toward the developing sleeve 2, whereby the developer is moved in the direction of the arrow "d" along the peripheral surface of the developing sleeve 2, with the developer taking the form of a magnetic brush on the developing sleeve 2. In accordance with the clockwise movement of the developer, in the form of the magnetic brush, the developer is in rubbing contact with the electrostatic latent image which has already been formed on the photoreceptor surface in a known manner so that the developer is used for developing the electrostatic latent image on the photoreceptor surface. After having accomplished the developing step, the developer is further moved in the direction of the arrow "d" and then scraped off the peripheral surface of the developing sleeve 2 by the scraping member 10 and is stirred and mixed by a developer stirring mechanism (not shown). Thereafter, similar movements of the developer are repeated by the operation of the bucket roller 12.
The foregoing description relates to the magnetic brush developing apparatus proposed by the present inventors and disclosed in the U.S. Patent application Ser. No. 74,812 mentioned earlier.
In the various developing experiments carried out by the present inventors over a long period on the above magnetic brush developing apparatus for detailed analysis of its adaptability with respect to the developer described earlier, favorable results have been confirmed in many respects. However, it has also been observed that said magnetic brush developing apparatus still has some deficiencies as follows. It is to be noted that, for the above experiments, the developer disclosed in the U.S. Patent application Ser. No. 949,426 mentioned earlier was employed.
More specifically, upon repetition of the developing experiments over a long term on the above magnetic brush developing apparatus, it was observed that, at the side of the scraping member 10 remote from the developing sleeve 2, i.e. at a position (B) shown in FIG. 1, an accumulation or pool of stagnant developer was gradually formed, which hindered the movement of the developer at the developing section. According to the analysis made by the present inventors, the formation of the pool of developer is surmised to be mainly attributable to the fact that, since the developer at the position (B) is subjected to a magnetic force exerted upwardly in FIG. 1 due to the magnetic action of the multipolar magnet member 3 which is balanced by gravity exerted downwardly in FIG. 1 by the weight of the developer itself, it is extremely difficult for the developer to be subjected to the moving force due to the rotation of the multipolar magnet member 3, and said developer can not readily be moved at the position (B). The obstruction to the smooth movement of developer at the developing section as described above also tends to bring about such disadvantages as occurrence of fogging in the resultant images after the development, and conspicuous scattering of the developer out of the developing apparatus through the gap between the surface 1a of the photoreceptor drum 1 and the lower closing member 7.